Corrugated thermoplastic blank for a container

ABSTRACT

This invention relates to a corrugated thermoplastic blank for a container. The blank (20) has a base panel (24) with wall panels (26, 28, 30, 32) connected to the base panel (24) by fold lines (38). The fold lines (38) comprise weld lines which destroy the original memory of the thermoplastic material. The free edges of the blank are sealed by seals (38) which prevent water from entering the blank. The sealed free edges and/or the weld lines or line (38) surround and reinforce each panel (26, 28, 30, 32) and increase the strength of each panel and thereby the strength of a container erected from the blank.

TECHNICAL FIELD

This invention relates to a corrugated thermoplastic blank for producinga container.

BACKGROUND OF THE INVENTION

Corrugated thermoplastic sheet has proved to be unsuitable for theconstruction of boxes especially where relatively heavy loads are to becarried in such boxes. These relatively heavy loads may comprise fish orvegetables which weigh in the order of 4 to 15 kilograms. Boxesconstructed from the corrugated thermoplastic sheet have been too weakfor these loads. Such corrugated thermoplastic sheet has traditionallybeen 3 to 4 mm thick and has had a weight of approximately 600 grams perm² with a buckling load resistance index of about 100. This sheet couldsimply be made thicker and heavier to increase its strength. However byincreasing the thickness and the weight of the sheet, more material isused and consequently it is not cost effective to increase the thicknessand the weight of the sheet to increase the strength of the sheet.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a lightweight yetsufficiently strong blank made of corrugated thermoplastic sheet.

The applicant has addressed the above objective by increasing theoverall thickness of the sheet whilst using substantially the sameamount of material as used in prior an sheets. The skins and flutes ofthis sheet are thus thinner than the prior art sheets. However suchsheet is unsuitable for the construction of the required boxes since theflutes can collapse as the skins move relative to one another.

It is accordingly a further object of this invention to provide areinforced lightweight, corrugated thermoplastic blank which can be usedto produce a box of sufficient strength.

Corrugated thermoplastic boxes have suffered from the disadvantage thatwater can be trapped between the two skins of the corrugated sheet. Suchwater, besides adding to the weight of a box, can promote the growth ofbacteria which is obviously undesirable especially in food applications.

The thermoplastic material, especially polypropylene, has a memory whichis not destroyed by the creation of fold lines which are created in thesame manner in which fold lines are created in corrugated cardboardblanks. Thus even after a fold line has been created on a sheet ofpolypropylene in the traditional corrugated cardboard manner, the sheetis not easily folded along that line because it tends to return to itsoriginal unfolded planar shape because of its memory. It is importantthat the walls of a box erected from the blank be perpendicular to thebase of the box. If the original memory of the thermoplastic sheet isnot destroyed the sidewalls of the box will tend to move out of theirperpendicular orientation and thus decrease the stacking strength of thebox.

It is accordingly a further object of this invention to provide acorrugated thermoplastic blank for a box in which water cannot becometrapped within the corrugated sheet and wherein the original memory ofthe thermoplastic material is substantially reduced or destroyed alongthe fold lines of the blank.

According to the invention a container blank made of corrugatedthermoplastic sheet having two skins spaced from one another by flutesincludes a base panel connected to wall panels by fold lines eachcomprising a weld line welding the two skim together and substantiallyreducing or destroying the original memory of the thermoplastic alongthe length of the weld line, and wherein the free edges of the blank aresealed to prevent liquid from entering the blank with the weld lines andthe sealed free edges and/or the weld lines surrounding and reinforcingeach panel thereby increasing the strength of each panel of the blankand thereby the strength of a container erected from the blank.

The sheet is preferably at least 5 mm thick but not thicker than 10 mmand may be between 6,0 mm and 7,5 mm thick.

The sheet may weigh between 400 and 850 grams/m². Preferably the sheetweighs between about 500 grams/m² and about 700 grams/m².

The panels of the blank preferably have a buckling load resistance indexof between 300 and 500.

The blank may include a plurality of apertures in at least one of thepanels with the edges of the apertures being sealed.

At least one of the panels my include at least one weld within itsperiphery which increases the strength of the panel.

The thermoplastic is preferably polypropylene.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of a non-limiting examplewith reference to the accompanying drawings in which:

FIG. 1 is an end view of part of a sheet of corrugated polypropylenesheet used for forming a blank;

FIG. 2 is a plan view of a blank for forming a box;

FIG. 3 is a perspective view of a box formed from the blank of FIG. 2;

FIG. 4 is a cross-sectional view on line IV--IV of FIG. 2;

FIG. 5 is a cross-sectional view on line V--V of FIG. 2;

FIG. 6 is a cross-sectional view on line VI--VI of FIG. 2; and

FIG. 7 is a cross-sectional view on line VII--VII of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring firstly to FIG. 1, an extruded sheet 8 of corrugatedpolypropylene consists of two skins 10 spaced from one another by flutes12. The overall thickness of the sheet is 7 mm as indicated by referencenumeral 14 and the weight of the sheet is in the order of 600 grams perm².

Referring now to FIGS. 2 to 6, the sheet 8 is die cut by heat andpressure to form a blank 20 and two end sections 20.1 for a box 22. Theblank 20 has a base panel 24, side panels 26, end panels 28, obliquepanels 30 and flaps 32. The end sections 20.1 of the blank each includean end panel 34 and two flaps 36.

When the blank 20 and the end sections 20.1 are die cut, their freeedges are sealed by seals 38 as shown in FIG. 4. The sealed free edgesprevent water from entering the blank and becoming trapped within theblank.

As shown in FIG. 5, fold lines 38 separate the panels from one another.The fold lines 38 are weld lines which weld the two skins 10 together.The original memory of the polypropylene is substantially destroyedalong these weld lines.

Each panel is surrounded by its sealed free edges and/or its weld lineor lines which reinforce the panel and increase its strength especiallywith respect to its resistance to buckling, shear and torsional forces.

Holes 40 are formed in the base panel 24. The edge of each hole issealed by a seal 42 which connects the skins 10 together. The applicanthas found that the holes with their sealed free edges increase thestrength of the panels with regard to resistance to buckling, shear andtorsional forces acting across the flutes. The holes 40 also act asdrainage holes.

Internal circular welds 44 are provided on the base panel 24. Thesecircular welds 44 weld the two skins 10 together. The circular welds 44could also be provided in the other panels of the blank. The applicanthas found that these circular welds strengthen the panels with regard tobuckling, shear and torsional forces acting across the flutes. Thesewelds 44 need not be circular but may be of any suitable shape and canbe arranged to dissipate stress dependent on their pattern, size andposition.

The blanks 20 and the end sections 20.1 are erected to form the box 22which is shown in FIG. 3.

The applicant has found that a corrugated polypropylene blank with athickness in the range of 5 mm to 10 mm with a weight in the range of400 to 850 grams/m² provides a lightweight box which is stronger thanprior art boxes for a given weight. The panels of the blank according tothe invention have a buckling load resistance index of between 300 and500.

It will be appreciated that many modifications or variations of theinvention are possible without departing from the spirit or scope of theinvention. For example the corrugated sheet could be made frompolyethylene or any other suitable olefin. The flutes may be vertical,sinusoidal, double sinusoidal or inclined relative to their skins.

We claim:
 1. A container blank made of corrugated thermoplastic sheethaving two skins spaced from one another by flutes, said blank includinga base comprising a weld line welding the two skins together andsubstantially reducing or destroying the original memory of thethermoplastic along the length of the weld line, and wherein the blankincludes edges being sealed to prevent liquid from entering the blankand surrounding and reinforcing each panel thereby increasing thestrength of each panel of the blank and thereby the strength of acontainer erected from the blank, the corrugated thermoplastic sheetbeing between 5 mm and 10 mm thick and having a weight of between 400grams/m² and 850 grams/m².
 2. The blank of claim 1 wherein thecorrugated thermoplastic sheet is between 6.0 mm and 7.5 mm thick. 3.The blank of claim 1 wherein the corrugated thermoplastic sheet has aweight of between 400 grams/m² and 700 grams/m².
 4. The blank of claim 1wherein the panels of the blank have a buckling load resistance index ofbetween 300 and
 500. 5. The blank of claim 1 including a plurality ofapertures including edges in at least one of the panels, the edges ofthe apertures being sealed.
 6. The blank of claim 1 wherein at least oneof the panels includes a periphery and further includes at least oneweld within said periphery thereof which increases the strength of saidat least one of the panels.
 7. The blank of claim 1 wherein thethermoplastic is polypropylene.
 8. The blank as claimed in claim 1wherein said blank is erected into a form of a container in the shape ofa box.
 9. A container as set forth in claim 1 wherein said sealed freeedges surround and reinforce each panel.
 10. A container as set forth inclaim 1 wherein said weld lines surround and reinforce each panel.